The reaction of aromatic dianhydrides such as benzophenone tetracarboxylic dianhydride (BTDA) with aromatic diamines to produce a polyimide acid which can then be heated to produce a polyimide is well known in the art. Such polyimides are useful as adhesives, molding powders and films having excellent chemical resistance and thermal resistivity properties. U.S. Pat. No. 4,065,343 to Donald J. Proger describes a method for preparing an adhesive solution of a polyimide acid by reacting an aromatic dianhydride with an aromatic diamine in the presence of certain specified ether solvents including diglyme and other aliphatic acyclic ethers. Proger teaches in column I, lines 57 et seq., that "In certain instances, a highly viscous polyamic-acid intermediate polymer precipitates from the ether solvents. The addition of very small amounts of water or alcohol generally leads to redissolution of a polyamic-acid to give a viscous polymer solution."
We confirm the above findings when diglyme, a preferred solvent due to availability and boiling point, is used. The two-step technique of Proger suffers from the fact that it is a two-step process but more particularly because the precipitated polymer severely limits the type and size of equipment which can be employed commercially. Proger in his Example II (columns 5 and 6), which uses diglyme as the solvent, employs a household blender to perform the experiment. The normal paddle-stirred devices cannot be employed as the precipitated polymer tends to wrap itself around the paddle blades resulting in potential damage, stoppage and other problems such as an increased energy usage. All of the above problems are overcome using the method of this invention.
Proger also teaches that in some instances, a precipitate does not form, but it is not clear from Proger's teachings which solvent or combination of solvents will result in a single phase solution polyimization reaction medium. The formulation of a polymer precipitate is desirably avoided since the precipitate causes stirring problems or difficulties which eventually limit the size of the reactor and the type of stirring mechanism which can be employed.
It is therefore desirable to have a single solvent for the production of an aromatic polyimide acid which allows for a single phase solution polymerization reaction to occur to produce aromatic polyimide acids having the desired molecular weight.